The chromosomes of 50 idiopathic mentally retarded patients with at least three other anomalies and 50 normal subjects were analyzed from randomized coded slides. The chromosomally abnormal cases were also studied by means of Q-banding. Seven of the patients showed a chromosome anomaly. Two had an extra G-like chromosome, which in case M99 consisted mainly of 16p (he had also a small Y chromosome shared by his father and brother), and in case M8 of 9p. The mother and sister of M99 were balanced translocation carriers. Case M18 (published separately) has approximately half of 12p deleted. Case M60 had a deletion of 20q and trisomy for a segment which most probably came from 7p. Case M49 had a deletion of 1p and increased centric heterochromatin in 1q: the latter abnormality was shared by the father and a sister. Case M83 displayed low-grade mosaicism for cells with an extra small ring of unknown origin. Case M38 had the brightly fluorescent distal part of the Y chromosome duplicated, and the father had the same chromosome. In the five first cases the phenotype was presumably caused by the chromosome anomaly, and in the mosaic this is a possible cause. In the normal subjects, two persons had minor chromosome anomalies: case M68 had a pericentric inversion in the Y chromosome, which was found also in the father and the brother, and case M10 had a telocentric no. 21. Three of the patients (M18, M49 and M99) can be regarded as type specimens for new syndromes in the sense that the chromosome anomalies causing the respective phenotypes have been identified for the first time.